ULTRASTRUCTURAL CHANGES IN DIRECTLY GERMINATING SPORANGIA OF PHYTOPHTHORA PARASITICA

Flagella and cleavage vesicles form during the initial stages of direct germination in the same manner as they do in indirect germination. Later, however, the flagella degenerate and cleavage of the cytoplasm is not complete. Instead, a new wall layer is deposited onto the existing sporangial wall, and this germination wall extends with the germ tube and forms the hyphal wall. Material for the germination wall first appears around vesicular aggregates concentrated at the periphery of the sporangial cytoplasm. As the wall forms the vesicular aggregates become encased in it, and the wall eventually consists of irregular deposits of wall material interspersed with the vesicular aggregates. These are clearly cytoplasmic in nature and different from lomasomes, since they are associated with ribosomes and other small elements of the cytoplasm. Likely to contribute to the wall formation are the endoplasmic reticulum, microbodies, dictyosome-derived vesicles, and possibly polyvesicular bodies and larger vacuoles with fibrillar contents. The basal plug of the sporangium forms the same way as the germination wall and also contains numerous vesicular inclusions. Fewer vesicular inclusions are found in the sporangial wall, and none have been observed in the walls of the growing hyphae. This difference in wall structure can be explained on the basis of their different growth patterns. Penetration of the sporangial plug is concomitant with prominent lomasomal activity and the cytoplasm at the hyphal tip is characterized by the presence of numerous vesicles, which are probably derived from dictyosomes.